1. Field of the Invention
This invention relates to a highly efficient coding apparatus applied to the reduction of the transmission data amount of a digital picture signal such as a digital television signal.
2. Description of the Prior Art
The present applicant has proposed an adaptive coding apparatus for obtaining a dynamic range which is equal to a difference between the maximum value and the minimum value of plural picture elements contained in a two-dimensional block and performing encoding adaptive to the dynamic range, as described in Japanese Patent Unexamined Publication Sho 61-144989. Also, disclosed in JPUP Sho 62-92620, an adaptive coding apparatus has been proposed for carrying out encoding adaptive to a dynamic range with respect to a three-dimensional block formed by picture elements of areas which are contained in each of plural frames. Further, as described in JPUP Sho 62-128621, a variable length encoding method for varying the bit number depending on a dynamic range to make constant the maximum distortion occurring at the time of quantization has been proposed.
The above-stated encoding adaptive to the dynamic range (called "ADRC") enables the decrease of the bit number per picture element by compressing a redundancy in the level direction and can thereby compress the data amount to be sent to a great extent. Such ADRC is suited for a digital VTR. Moreover, for the purpose of compressing the transmission data amount with the elimination of the redundancy in the time direction, the present applicant has proposed a hybrid system in which adaptive frame dropping for transmitting an average value of picture elements in the three-dimensional block in the case of a block of a still picture and ADRC for detecting a movement of a picture in a block in the case of the ADRC of the three-dimensional block are combined (refer to Patent Application Sho 60-247840).
Adaptive frame dropping of the three-dimensional block will be described referring to FIG. 1. In FIG. 1, F1, F2, F3, . . . represent a continuous first frame, second frame, third frame, . . . The three-dimensional block is composed of, for example, two areas corresponding in space and respectively belonging to frames continuous in time which are obtained by the division of one frame picture into many two-dimensional areas. A level change of one picture element data at the same position in space of the three-dimensional block is shown in FIG. 1.
One block is constructed by two areas respectively belonging to the frames F1 and F2, and, similarly, one block is composed of two areas respectively belonging to frames F3 and F4, frames F5 and F6, . . . In these blocks, when the change of the luminance in the time direction of picture element data a0 and a0', a1 and a1', . . . is slow, it tends to be decided as a block of a still picture (called "still block"). In the still block, an average of picture element data corresponding to each other in space of the values such as 1/2 (a0+a0'), 1/2(a1+a1'), . . . are formed as indicated by black dots. These average values are sent in place of original picture element data. On the reception side, the average values are used repeatedly over the two-frame period for use as the reproduced picture element data. As a result, the number of transmission picture elements is reduced to 1/2 when the frame dropping is perform.
In this way, when the level change of picture element data at the same position in space is slow, it tends to be decided as a still block. Moreover, when the decision is sensitive with respect to a still block and a block of a moving picture (called "a moving block"), there is a case where one of two adjacent blocks in space is decided as a still block and the other is decided as a moving block. For example, in FIG. 1, there is a case where the block containing a0 and a0' is decided as a still block and a block adjacent to this block is decided as a moving block. In this case, there is no problem for the area where a difference of luminance level is inherently found, as in the contour of a thing. However, for the area where almost the same luminance level is seen, block distortion takes place to provide a level difference between adjacent blocks which is the cause of an unacceptable picture.